Method and device for measuring at least one characteristic length on a fuel rod arranged at the periphery of a nuclear fuel assembly

ABSTRACT

An eddy-current probe (10) is moved in the axial direction of the clad of the rod (1), a first position of the probe (10) is determined in the axial direction, for which a signal from the probe is representative of the presence of the internal surface of the bottom end plug of the rod (1) and at least one second position for which the signal from the probe is representative of the presence of the end part of the spring of the fuel rod (1) or of the internal surface of the top end plug of the fuel rod (1). The total length and/or the length of the fissile stack are calculated from the first position and from the second position of the probe (10). The measurements may be performed inside the spent fuel storage pond of a reactor.

FIELD OF THE INVENTION

The invention relates to a method and to a device for measuring acharacteristic length on a rod of a nuclear fuel assembly.

BACKGROUND OF THE INVENTION

Nuclear fuel assemblies for water-cooled reactors, and in particular fornuclear reactors cooled by pressurized water, include a framework inwhich the rods are held in place in the form of a bundle in which therods are all mutually parallel.

Each of the rods in the bundle of the nuclear fuel assembly includes atubular cladding whose diameter is approximately, for example, onecentimeter and whose length may be about four meters. The cladding ismade of a low neutron-absorbent material such as a zirconium alloy. Thenuclear fuel material, which may consist of uranium oxide enriched withfissile uranium, is arranged inside the cladding in the form of a stackof sintered pellets of fuel material which are stacked on top of eachother in the axial direction of the cladding. The cladding of the rod isclosed at a first end by an end plug engaged in the end part of thecladding of the rod and welded to the cladding. The pellet stack of fuelmaterial arranged inside the cladding rests with one of its ends on asubstantially plane surface of the end plug internal to the cladding.This first end plug of the fuel rod, on which the pellet stack of fuelmaterial rests, constitutes the bottom end plug of the rod when the rodis arranged inside a fuel assembly in the service position in the coreof a nuclear reactor or in the storage position in a pond.

A second end plug, called the top end plug, is engaged and welded to thesecond end of the cladding of the rod, this second end plug alsoincluding a surface directed towards the inside of the cladding. Inorder to allow a degree of expansion and a degree of swelling of thepellets of fuel material under the effect of the heating and the nuclearreactions inside the core of the nuclear reactor, the pellet stack offuel material has a length less than the length of the internal space inthe cladding of the rod between the internal surface of the bottom endplug and the internal surface of the top end plug. The pellet stack offuel material is held in place inside the cladding by a helical springinserted between the internal surface of the top end plug and the topend of the pellet stack opposite that end of the stack resting on thebottom end plug. When the nuclear reactor is in service, the fuel rodsare subjected to a neutron flux which produces energy in the fuelmaterial of the rods by nuclear reactions. The neutrons produced by thenuclear reactions in the pellets of fuel material constitute the neutronflux inside the core of the nuclear reactor.

Because of the nuclear reactions and the heating in the core of thereactor, the pellets of fuel material undergo expansions and swellingwhich may lead to deformation of the cladding of the rod. Furthermore,the thermal and mechanical stresses to which the fuel rods of thenuclear fuel assemblies in the core of the nuclear reactor are subjectedmay lead to damage which is added to the normal depletion of the fuel byburnup of the fissile material, so that at least some of the fuel rodsof the assemblies become unsuitable for use in the fuel assemblies ofthe core after a certain residence time in the core of the operatingreactor.

Periodically, the core of the nuclear reactor is refuelled, by replacingwith new assemblies some (generally a third) of the core assemblies andrepositioning the other core assemblies into new positions for a newperiod of operation of the nuclear reactor. To this end, after thenuclear reactor has been shut down, the fuel assemblies are removed fromthe core and placed in a fuel storage pond.

It is necessary to carry out various checks on the fuel rods beforerepositioning them in the core, these checks relating both to theframework supporting the rods of the assemblies and to the rodsthemselves.

The framework for the nuclear fuel assemblies includes, in particular,spacer grids distributed along the height of the fuel assemblies atregularly spaced-apart distances, which include a lattice of generallysquare-mesh cells in each of which is placed a fuel rod which is heldinside the cell by springs and by bearing dimples.

The cross-section of the fuel assemblies, generally of square shape,includes rows of rods arranged in two directions at 90°. Only the fourperipheral rows of rods are easily accessible from outside the fuelassembly and may be subjected to examination, such as visual examinationor examination using non-destructive test methods. These checks remainpurely qualitative and do not make it possible to obtain measurementsrelating to the deformation of the rods and to the state of the fuelmaterial and of the spring inside the claddings, after a certain periodof operation of the nuclear reactor.

The fact that it is not possible to obtain accurate numerical datalimits the effectiveness of the fuel test programs in a nuclear reactor.

It has been proposed to carry out checks on the rods of the fuelassemblies in checking stations inside the fuel storage pond, but inorder to carry out these checks it is necessary to dismantle the fuelassemblies and extract the rods in order to examine the irradiated rodsunder water.

It may be highly advantageous, with regard to knowing the behavior ofthe fuel rods in the core of a nuclear reactor, to measure certaincharacteristic lengths of a fuel rod after a residence time in thenuclear reactor. In particular, it may be extremely advantageous to beable to measure accurately the length of the pellet stack of fissilefuel material, or fissile stack, after a certain residence time in thecore of the reactor during operation, or else the total length of therod.

To date, there has been no known technique for precisely measuring thecharacteristic lengths on an irradiated fuel rod, in a fuel assemblyarranged under water inside a pond.

SUMMARY OF THE INVENTION

The object of the invention is to propose a method for measuring atleast one characteristic length on a fuel rod arranged at the peripheryof a nuclear fuel assembly, the fuel rod including a tubular cladding, astack of pellets of nuclear fuel material which are stacked in the axialdirection of the cladding, a first closure end plug or bottom end plug,at a first end of the cladding, in contact with a first end of the stackof pellets of fuel material by means of an internal surface, a secondclosure end plug, for closing the second end of the cladding, or top endplug, and a helical spring inserted between an internal surface of thesecond end plug and a second end of the stack of pellets of fuelmaterial, inside the cladding, this method making it possible todetermine very accurately a characteristic length such as the length ofthe fissile column of the rod or the total length of the rod, withoutdismantling the fuel assembly and without extracting the rods.

To this end:

an eddy-current probe is moved in the axial direction of the cladding ofthe rod;

a first position of the eddy-current probe, with respect to a marker, inthe axial direction, is determined, in which position a signal from theprobe is representative of the presence of the internal surface of thefirst end plug level with the probe and at least one second position inwhich the signal from the probe is representative of the presence, levelwith the probe, of one of the following elements: the end part of thespring, in contact with the second end of the stack of pellets, and theinternal surface of the second end plug; and

the characteristic length is calculated from the first position and fromthe second position of the probe, these positions being defined withrespect to the marker.

The invention also relates to a device making it possible to use themeasurement method according to the invention inside a spent fuel pondof a nuclear reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, adescription will now be given, by way of example, with reference to thedrawing figures appended hereto, of one embodiment of the methodaccording to the invention which makes it possible to determine thelength of the fissile stack and the total length of any peripheral rodof a fuel assembly inside the fuel pond of a pressurized-water nuclearreactor.

FIG. 1 is a view in section through a vertical plane of a rod of a fuelassembly of a pressurized-water nuclear reactor.

FIG. 2 is an elevation of an eddy-current device making it possible touse the method according to the invention.

FIG. 2A is a schematic plan view of FIG. 2.

FIG. 3 is a schematic view of the length measurement system associatedwith the device shown in FIG. 2.

FIG. 4 is a plan view in the direction of arrow 4 in FIG. 5 of theeddy-current probe of the measurement device shown in FIG. 3, in contactwith a peripheral fuel rod of a fuel assembly.

FIG. 5 is a side view along the direction 5--5 in FIG. 4.

FIG. 6 is an axial sectional view of the eddy-current probe shown inFIGS. 4 and 5.

FIGS. 7A, 7B and 7C represent graphical recordings obtained while themeasurement method according to the invention is being used.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 shows a fuel rod 1 of a nuclear fuel assembly of apressurized-water nuclear reactor. The fuel rod 1 includes a tubularcladding 2 made of a zirconium alloy such as Zircaloy 4, having adiameter of about one centimeter and a length of about four meters. Forease of representation in FIG. 1, the proportions of the rod have beengreatly changed, the ratio of the diameter to the length of the rodhaving been greatly increased.

A first end plug 3, or bottom end plug, is engaged in a first end partof the cladding 2 which is closed by the end plug 3. The end plug 3includes a cylindro-frustoconical part internal to the clad, the maximumdiameter (diameter of the cylindrical part) of which is approximatelyequal to the inside diameter of the cladding so that the end plug isengaged virtually without any clearance in the bottom end part of thecladding. That part of the end plug 3 arranged inside the cladding 2terminates in a plane surface 3a forming the small base of thefrustoconical end of the internal part of the end plug 3. The cladding 2contains a stack 4 of nuclear fuel pellets 5 which are stacked on top ofone another in the axial direction of the cladding 2. For the sake ofsimplicity, the stack 4 has been shown as consisting of a relativelysmall number of pellets of fuel material. In reality, the claddingcontains a very large number of pellets 5 stacked on top of one another.The pellets 5, obtained by sintering, may be made, for example, of afissile fuel material such as uranium oxide UO₂ enriched with uranium235. A first end of the stack of fuel material or fissile stack 4 restson the internal surface 3a of the bottom end plug 3.

Inserted into the second end part of the cladding 2 of the rod, oppositethe first end closed by the end plug 3, is a top end plug 6 which has ashape substantially identical to the bottom end plug 3 and whichincludes a cylindro-frustoconical part which is engaged inside thecladding 2 and terminates in a plane internal surface 6a substantiallyperpendicular to the axis of the cladding 2.

The total length in the axial direction of the stack 4 of pellets 5 offissile combustible material is less than the length in the axialdirection of the cladding between the internal surface 3a of the bottomend plug 3 and the internal surface 6a of the top end plug 6.

A helical spring 7 is inserted between the internal end 6a of the topend plug 6 and the top end of the fissile stack 4 opposite the end ofthe stack resting on the internal surface 3a of the bottom end plug 3.The spring 7, which is compressed between the end 6a of the end plug 6and the top end of the fissile stack 4, holds the fuel pellets in placeinside the cladding 2 of the rod 1.

After fitting the end plug 3, the fissile stack 4 is produced bystacking the pellets of fissile fuel material, the base of the stackresting on the internal surface 3a of the end plug 3. Next, the helicalspring 7 and the top end plug 6 are fitted and the end plugs are weldedto the cladding. The internal volume of the cladding 2 is evacuated andfilled with an inert gas via the top end plug 6 which is subsequentlyclosed in a sealed manner.

The fissile fuel pellets 5 have a diameter less than the inside diameterof the cladding so as to allow radial expansion of the pellets 5 duringthe irradiation-induced swelling of the pellets in the nuclear reactorduring operation.

The axial expansion of the fissile stack 4 is compensated for by thehelical spring 7.

After a certain period of use of a fuel assembly in the core of anuclear reactor, the rods, which are engaged in the framework of theassembly, may have deteriorated somewhat, this being manifested by adeformation of the cladding 2 and by a modification in the shape andlength of the fissile stack 4.

In order best to determine the behavior of the fuel rods in the reactorduring operation, it may be necessary to perform accurate measurementsof characteristic lengths of the rod, and in particular measurements ofthe total length of the rod (length L_(T) in FIG. 1) or measurements ofthe length of the fissile stack 4 (L_(FS) in FIG. 1).

It may be extremely advantageous to perform these measurements in thefuel cooling pond, without dismantling the fuel assemblies.

The method and the device according to the invention make it possible toperform such measurements in the cooling pond on the peripheral rods ofthe fuel assemblies removed from the core and stored in the spent fuelpond.

In order to perform the measurements of characteristic lengths on theperipheral rods of a fuel assembly, using the method according to theinvention, a device such as that shown in FIGS. 2 and 2A may beemployed.

The measurement device 8 includes an eddy-current probe 10 carried by ahandling assembly 9 which will be described below, itself resting on acarrier 11 which may be fastened to the compartment 12 of an elevatorfor the spent fuel pond of the nuclear reactor.

As may be seen in FIGS. 2 and 2A, which show the measurement device 8 inthe service position inside the spent fuel storage pond of a nuclearreactor, the wall 13 of the pond includes, inside a housing 14 set backinside the wall 13 of the pond, an elevator 12 consisting of acompartment which can accommodate a fuel assembly 15 and is associatedwith means for moving it in the vertical direction inside the verticalhousing 14 in the wall of the pond in order to lower a fuel assembly 15into the bottom of the pond. The fuel assemblies may be handled by thehandling crane of the spent fuel pond in order to be laid in fixedcompartments resting on the bottom of the spent fuel pond.

The measurement device 8 according to the invention is used inconjunction with a spent fuel pond elevator, the carrier 11 includingmeans enabling it to be clamped to the upper part of the compartment 12of the spent fuel pond elevator.

The carrier 11 includes a horizontal plate on which rests a table 16 formovement in two directions at 90° in the horizontal plane. The table 16or XY table supports a vertical guide column 17 on which is mounted, forvertical movement along the axis 18, a carriage 19 for moving the probe10 vertically.

The device which has just been described makes it possible to place theprobe 10 opposite any rod lying at the periphery of the assembly 15,such as the rods 1a, 1b or 1c shown in FIG. 2A, by moving the XY table16 in its first horizontal movement direction X.

The second movement in the horizontal direction (movement in thedirection Y) makes it possible to move the eddy-current probe 10perpendicularly to one face of the fuel assembly 15 between a positionin which the probe 10 comes into contact with a fuel rod 1 of theassembly 15, as shown in FIG. 2, and a set-back position in which theprobe 10 is not in contact with a rod of the assembly, as shown in FIG.2A.

Motor means 22 controlled remotely from the edge of the spent fuel pondmake it possible to move the measurement device 8, including the probe10, in the X and Y directions; using a motor means 21, the probe 10 maybe moved in the vertical direction or moved in the Z direction veryprecisely and at a very slow speed.

The probe 10 may also be moved vertically at high speed, the probe 10being in its set-back position away from the peripheral rods of the fuelassembly 5, by virtue of the elevator 12 to which is clamped the carrier11, on which the measurement device 8 with its movement means ismounted. Moving the entire measurement device 8 at high speed, using theelevator, makes it possible to place the probe 10 approximately oppositea measurement zone of a fuel rod of the assembly 15, such as the bottomend plug, the top end plug or the bottom part of the spring holding thefissile stack in place. When the probe 10 has been positionedapproximately opposite the measurement zone, the elevator is stopped andthe position of the probe 10 in the vertical direction is finelyadjusted after it has been brought into contact with the rod 1 on whichthe measurement is to be performed, by moving the carriage 19 at verylow speed.

During movements of the probe 10 in the vertical direction, whetherthese movements are carried out at high speed or at low speed, theposition of the probe 10 in the direction Z is determined veryaccurately by using a marker 20 placed vertically along the wall of thefuel pond in an arrangement adjacent to the vertical housing 14 of theelevator.

The marker 20 consists of a flexible rule housed in a winder boxarranged at the upper level of the pond, one end of which is fixed tothe carriage 19 for moving the eddy-current probe 10 in the direction Z,including gradations which may be read and counted by a coding device25. The coding device 25, arranged at the upper level of the pond,provides an output signal having square pulses, each of the pulses ofthe coder signal corresponding to a vertical distance of 0.1 mm. It istherefore possible to know the position of the probe with an accuracy of0.1 mm.

Two video cameras 23 and 24 are designed to provide, at the upper levelof the pond, on a screen, a picture of the end part of the probe 10 inorder to verify that it has been put into place in the service on one ofthe peripheral tubes 1 of the assembly 15. The camera 23 provides apicture of the eddy-current probe 10 from above and the camera 24provides a picture of the end of the probe 10 in a side view.

The carrier 11 of the measurement device 8 with its means for movementin the directions X, Y and Z includes a support belt having a squareinternal opening allowing passage of a fuel assembly as well as catches26 for fixing the carrier 11 to the top part of the compartment 12 ofthe elevator, the catches 26 being associated with operating deviceswhich may be actuated from the upper part of the spent fuel pond. Twoguide rollers 27a and 27b are mounted on the top surface of the belt ofthe carrier 11 by means of pivoting supports so as to guide themovements of the carrier 11 with respect to the substantially planeexternal faces of the square-section assembly 15. The supports for theguide rollers 27a and 27b are pivotally mounted so as to allow therollers to pass over the grids 28 of the assembly 15, in which grids therods of the assembly are engaged, the rods thus being held in a uniformlattice.

The eddy-current probe 10 is mounted on a support fixed to thevertically moving carriage 19 and connected by means of a measurementcable 29 to a device, arranged at the upper level of the pond, forsupplying electric current and for making use of the measurements. Thecameras 23 and 24, as well as the movement devices 21 and 22, are alsoconnected to supply and control means arranged at the upper level of thepond. It is therefore possible, remotely, to supply the probe, selectthe measurement signals, visualize the end part of the probe and therods during examination, and control the movements of the probe in theX, Y and Z directions.

The rapid movements of the probe in the direction Z are controlled bythe means for controlling the fuel pond elevator.

FIG. 3 schematically shows the measurement system which is associatedwith the measurement device shown in FIG. 2 and is arranged at the upperlevel of the pond. The measurement system includes a unit 30 called theeddy-current apparatus which combines the means for supplying theeddy-current probe and for collecting the measurement signals via thecable 29 as well as a unit 31 for visualizing the signals from theeddy-current probe and a graphics recorder 32 for recording thesesignals.

The eddy-current apparatus 30 has two output channels, called the Xchannel and Y channel, connected in parallel to the visualization unit31 and to the recorder 32.

In addition, the measurement system includes a display device 33connected to the height coder 25 of the measurement device, making itpossible to display to within 0.1 mm the height of the eddy-currentprobe 10 inside the spent fuel pond. A microcomputer 34 makes itpossible to manage the entire measurement program and to record theresults of the measurements on the various peripheral rods of the fuelassemblies on which the measurements of characteristic length arecarried out.

The control and measurement station located at the upper level of thepond also includes visual display screens which allow the operatorresponsible for the measurements to be provided with pictures of the endof the probe coming from the video cameras 23 and 24.

FIGS. 4 and 5 show the eddy-current probe 10 whose end part 10a formingthe measurement head is placed in contact with a fuel rod 1 of thebundle of the nuclear fuel assembly 15 on which length measurements arecarried out.

The measurement head 10a of the eddy-current probe 10 has a recessedshape with a substantially cylindrical contact surface allowing perfectcontact and perfect coupling between the measurement head and thesurface of the cladding of fuel rod 1.

The measurement head 102 may be made of zirconium oxide ZrO₂ orzirconia.

FIG. 6 shows the probe 10 whose measurement head 10a forming the endpart coming into contact with the cladding of the fuel rod 1 has ahousing for accommodating an end part of a measurement coil which has aferrite core and two windings 35 and 35' which are wound onto the coreso as to be coaxial and are spaced apart in the horizontal direction.The second end of the coil, on the winding 35' side, is engaged in acavity in a support 36.

The measurement coil 35, 35', the measurement head 10a and the support36 are mounted inside a movable support 37 which is mounted for slidingmovement in the axial direction 38 of the probe inside a main probe body39. The movable support 37 and the main body 39 of the probe are ofcylindrical shape and have as common axis the axis 38 of the probe. Themain probe body 39 includes an axial longitudinal groove 40 in which aguide pin 41 integral with the movable support 37 is engaged. The guidepin 41 is engaged with a certain clearance in the circumferentialdirection, inside the groove 40. In this way, the movable support 37 canpivot about the axis 38 to a limited extent only. The measurement head10a and, in particular, the end in the form of a cylindrical cavity thusmaintain an orientation with respect to the vertical rods 1 of thebundle of the nuclear fuel assembly which allows engagement of thecavity around a rod in order to perform the measurements, with a certainfreedom of rotational movement in order to adapt to the profile of therod.

Fixed inside the movable rod 37 is a thrust piece 42 which includes acavity having a V-shaped cross-section directed toward the measurementcoil 35, 35'. A standard reference tube 44 consisting of a portion of afuel rod cladding similar to the fuel rod 1 is inserted between thethruster 42 and the support 36 for that end part of the winding 35' ofthe coil opposite the measurement head 10a.

An annular stop piece 43 is screwed into the end part of the main body39 of the probe 10. A helical spring 45 is inserted between the stoppiece 43 and the thrust piece 42. The helical spring 45 allows movementsof the movable support 37 and of the measurement head 10a of the probe10 with respect to the main body 39 which is integral with the probesupport, both during contacting and during movements of the probe 10 inthe vertical direction. The spring 45 exerts a thrust on the movablesupport 37 and on the measurement head 10a in such a way that themeasurement head 10a is in perfect contact, by means of its cylindricalend surface, with the external surface of the cladding of the rod 1 onwhich the measurements are performed. This arrangement makes it possibleto avoid exerting, via the probe, excessive force on the cladding of therod and via the rod on the springs holding the rod in place in the cellsof the grids of the fuel assembly. Any plastic deformation of thesprings for holding the rods of the assembly in place during themeasurements performed on the peripheral rods is thus avoided.

To this end the carriage for moving the measurement device 8 in thedirection Y, driving the probe 10 forward toward the peripheral rods 1of the fuel assembly includes a front stop in a position such that, inits most forward position, the carriage for movement in direction Ycauses the measurement head 10a of the probe 10 to come into contactwith the fuel rod 1 and the spring 45 to be compressed such that theforce exerted by the measurement head 10a of the probe on the rod 1 isat most 10 N.

Furthermore, should the stop device of the carriage for moving in thedirection Y become defective, the spring 45 is provided with a constantsuch that the force exerted on the measurement head by the fullycompressed spring, i.e. when the guide finger 41 has come into abutmentagainst the bottom of the groove in the main body 39, is at most 25 N.

The measurement head 10a in its upper part and in its lower part has, asmay be seen in FIGS. 5 and 6, two chamfered contact surfaces inclined at45° with respect to the axis 38 of the probe. While the probe 10 incontact with a peripheral rod of a fuel assembly is being movedvertically, the inclined parts of the probe allow the probe easily tocircumvent the grids of the assembly, the peripheral belt of whichprojects slightly from the cladding of each peripheral rod of theassembly. The circumvention is achieved by the chamfered surfaces of themeasurement head 10a coming into contact with the peripheral belt of thespacer grid, in particular at guide fins of the fuel assembly whoseinclination is identical to the inclination of the chamfered parts ofthe measurement head 10a, and by a rearward movement of the movablesupport 37 of the probe which is accompanied by compression of thespring 45.

The measurement windings 35 and 35', which are arranged in the axialextension of one another, are supplied with alternating currentcharacteristically during the measurements, and are moved into thevicinity of the external surface of the fuel rod on which themeasurements are performed. The variations in the eddy currents flowingin the fuel rod opposite the probe lead to variations in the impedenceof the probe, allowing certain characteristic elements inside the cladof the rod to be detected depending on the signal provided by theeddy-current probe.

In particular, it has been possible to identify characteristic signalswhen the probe is located opposite the internal surface of the bottomend plug or the top end plug and also when the probe is located oppositethe bottom part of the spring bearing on the stack of fissile pellets.

The characteristic signal may be obtained, depending on the situation,either on the X output channel or on the Y output channel of theeddy-current apparatus.

FIGS. 7A, 7B and 7C show the characteristic signals obtained on the Xoutput channel and on the Y output channel of the eddy-currentapparatus, with respect to the graduated scale along the verticaldirection showing the height of the probe within 0.1 mm.

The signals shown in FIGS. 7A, 7B and 7C are obtained on the screen ofthe visual display unit 31 of the measurement system.

FIG. 7A shows the signals 50 and 51 coming from the X and Y outputs ofthe eddy-current apparatus 30 when the probe 10 is located opposite theinternal surface 3A of the bottom end plug 3 of a fuel rod 1, as shownin FIG. 1.

The signal 51 of the Y channel of the eddy-current apparatus has a verypronounced maximum value which makes it possible to determine veryaccurately the height of the internal surface 3a forming the bottom endof the fissile stack 4.

FIG. 7B shows the output signals 52 and 53 on the X and Y channels ofthe eddy-current apparatus, respectively, when the probe locatedopposite the bottom end of the spring 7 of the rod 1 coming into contactwith the top end of the fissile stack 4. The peak in the signal 52coming from the X channel of the eddy-current apparatus makes itpossible to determine with very great accuracy the position of thebottom end of the spring, i.e. the position of the top end of thefissile stack 4.

Finally, FIG. 7C shows the signals 54 and 55 which are respectively theoutput signals from the X and Y channels of the eddy-current apparatuswhen the probe 10 located opposite the internal surface 6a of the topend plug 6 of the fuel rod 1.

The peak in the output signal 55 of the Y channel makes it possible todetermine with very great accuracy the position of the internal surface6a of the upper end plug 6.

When the signals characteristic of the presence, opposite the probe, ofthe internal surface of the bottom end plug, the bottom end of thespring and the internal surface of the top end plug appear on the screenof the visual display unit, the height of the probe, given by the heightcoder, is detected and recorded.

Denoting the respective heights of the probe when the internal surfaceof the bottom plug, the bottom surface of the spring and the internalsurface of the top plug by Z_(B), Z_(S) and Z_(T) respectively, it ispossible to determine the total length of the rod L_(T) and the lengthof the fissile stack L_(FS) from the following formulae:

    L.sub.T =Z.sub.T -Z.sub.B +29.4 and

    L.sub.FS =Z.sub.S -Z.sub.B.

This is because the total length in the axial direction of the bottomend plug and the top end plug is known, this length being 29.4 mm.Moreover, the heights Z_(B), Z_(S) and Z_(T) in millimeters have been sodetermined that the formulae given above make it possible to determinethe total length of the rod and the length of the fissile column, inmillimeters, with an accuracy of about 0.1 mm.

In order to use the method according to the invention on all theperipheral rods of a fuel assembly stored in the bottom of the spentfuel pond, the handling crane of the spent fuel pond is employed inorder to take hold of the fuel assembly and bring it into alignment withthe axis of the compartment 12 of the elevator, to the top part of whichcompartment the carrier for the measurement device is clamped, as shownin FIG. 2.

The fuel assembly is oriented so that one of its peripheral faces isdirected perpendicularly to the axis 38 of the probe mounted on thecarriage 19 for moving the measurement device 8 vertically. Next, thefuel assembly is held in place in this position by the handling devicefrom which it is suspended.

The probe is placed in a set-back position obtained by rearward movementof the carriage for moving the XY table 16 in the Y direction. By movingthe XY table in the X direction, the axis of the probe 10 is placedopposite a first peripheral fuel rod of the assembly 15 arranged at oneend of the peripheral face directed towards the measurement probe.

By using the elevator, the probe 10 is approximately positioned oppositethe bottom end of the rod closed by the bottom end plug.

By means of a Y movement of the probe in the direction of advance, themeasurement head 10a of the probe 10 is brought into contact with theoutside surface of the cladding of the rod. The eddy-current probe ispowered and the signals produced by the probe during a very low-speedmovement in the vertical direction, using the carriage 19 for movementin the Z direction, are detected. The carriage 19 for movement in thedirection Z together with its guide and movement means are produced soas to obtain a maximum movement of the probe in direction Z at a speedof about 1 mm/second over a distance of from 100 to 150 mm.

When the characteristic signal, as shown in FIG. 7A, of the presence ofthe internal surface 3a of the bottom end plug 3 has been detected, theheight Z_(B) given by the height coder is recorded.

The probe is then moved back with respect to the guide tube by movementof the probe in the Y direction in order to move it away from the fuelrod.

The elevator is then used to move the measurement device 8 fixed to thecarrier 11 fastened to the compartment 12 vertically until theeddy-current probe reaches a high position which is close to thetheoretical high position of the top of the fissile stack 4. Detectionof the signal, as shown in FIG. 7B, representative of the bottom part ofthe spring 7 for holding the fuel pellets in place is then sought bymoving the probe vertically at low speed after having brought it backinto contact with the fuel rod 1 by a Y movement in the forwarddirection. Detection of the signal 52 representative of the bottom partof the spring makes it possible to determine the height Z_(S).

The probe is then placed in the set-back position again and the entiremeasurement device is moved until it is in the vicinity of the top partof the rod in order to determine the height Z_(T) of the internalsurface 6a of the top end plug 6.

It is therefore possible to determine the total length L_(T) of the rodor the length of the fissile stack L_(FS) or both these characteristiclengths of the rod by using the calculation formulae mentionedhereinabove.

Of course, all the operations for acquisition of the data relating tothe height of the probe and for calculation of the characteristiclengths of the fuel rod may be carried out automatically using themeasurement system shown in FIG. 3. The measured characteristic lengthsare also stored in a memory with respect to an identification number forthe rod of the assembly.

The same probe-movement, signal-detection and height-measurementoperations are carried out for each of the peripheral rods of that faceof the fuel assembly 15 located opposite the measurement apparatus.

Next, the fuel assembly suspended from the hoist of the handling cranemay be rotated through 90° in order to carry out the same operationssuccessively on each of the peripheral rods located on a second face ofthe fuel assembly.

The method according to the invention makes it possible to obtain,rapidly and extremely accurately, characteristic lengths of theperipheral fuel rods of irradiated fuel assemblies stored in the spentfuel pond of a nuclear reactor.

It is possible to measure characteristic fuel-rod lengths other than thetotal length of the rod and the length of the fissile stack, bydetecting signals characteristic of the presence of the probe opposite adefined region of the rod, located at a precise point along its length.

The measurement device may be moved along the length of the rod, and theprobe positioned with respect to the rods on which measurements areperformed, using devices other than an XY cross-movement table and acarriage for moving in the vertical direction Z.

Likewise, the measurement device may be moved rapidly along the rods ofthe assembly by a device other than a fuel assembly elevator.

The eddy-current probe may have a structure other than that which hasbeen described.

In some cases, it is possible to make corrections to the probe heightmeasurements, depending on the position of the rods along the peripheralface of the fuel assembly.

The invention may be applied to length measurements on peripheralnuclear fuel assembly rods of a type other than the nuclear fuelassemblies normally used in pressurized-water nuclear reactors.

What is claimed is:
 1. A method for measuring at least onecharacteristic length on a fuel rod (1, 1a, 1b, 1c) arranged at aperiphery of a nuclear fuel assembly, the fuel rod including a tubularcladding (2), a stack (4) of pellets (5) of nuclear fuel material whichare stacked in an axial direction of the cladding (2), a bottom end plugat a first end of said cladding and having an internal surface (3a) incontact with a first end of the stack (4) of pellets (5), a top end plug(6) for closing a second end of said cladding (2), and a helical spring(7) inserted between an internal surface (6a) of said top end plug (6)and a second end of the stack (4) of pellets (5) of fuel material,inside the cladding (2), said method comprising the steps of:(a) movingan eddy-current probe (10) in the axial direction of the cladding (2) ofthe rod (1); (b) identifying a first axial position of the eddy-currentprobe (10) with respect to a marker (20), in which position a signalfrom the probe (10) is representative of the presence of the internalsurface (3a) of the bottom end plug (3) level with the probe (10) and atleast one second position in which the signal from the probe (10) isrepresentative of the presence, level with the probe (10), of one of thefollowing elements: the end part of the spring (7), in contact with thesecond end of the stack (4) of pellets (5), and the internal surface(6a) of the top end plug (6); and (c) calculating the characteristiclength from said first position and said second position of the probe(10), these positions being defined with respect to the marker (20). 2.The method according to claim 1, wherein the signal from the probe (10)in said second position is representative of the presence, level withthe probe (10), of the internal surface (6a) of the top end plug (6),and including the step of calculating the total length (L_(T)) of thefuel rod (1) from a parameter (Z_(B)) representative of the position inthe axial direction of the internal surface of the bottom end plug (3),from a second parameter (Z_(T)) representative of the position in theaxial direction of the rod of the internal surface (6a) of the topsecond end plug (6) and from the sum of the lengths in the axialdirection of the bottom end plug (3) and the top end plug (6).
 3. Themethod according to claim 1 wherein, in said second position, the probe(10) is located opposite the bottom part of the spring (7), andincluding the step of calculating the length (L_(FS)) of the fissilestack (4) of the rod (1) from a parameter (Z_(B)) representative of theposition (10) of the internal surface (3a) of the bottom end plug (3)and from a second parameter (Z_(S)) representative of the position inthe axial direction of the bottom part of the spring (7) in contact withthe second end of the fissile stack (4).
 4. The method according toclaim 1, including placing the nuclear fuel assembly (15) in anarrangement in which the axial direction of the fuel rods is vertical,inside a spent fuel storage pond of a nuclear reactor.
 5. The methodaccording to claim 4, including arranging the fuel assembly along theaxis of an elevator placed in a vertical direction along a wall of aspent fuel storage pond of a nuclear reactor.
 6. The method according toclaim 1, including moving the eddy-current probe (10) at a first speedso as to place it in the vicinity of said first or second position withrespect to the peripheral fuel rod (1) of the fuel assembly (15), in theaxial direction of the fuel rod (1), and then moving said eddy-currentprobe at a second speed very much less than said first speed in thevicinity of said first or second position.